This invention relates to a valve control mechanism for an internal combustion engine, to engines containing such mechanisms and to a method of operating the valves of an internal combustion engine.
Our British Patent No. 2 190 140 describes and claims a valve control mechanism which comprises: a camshaft carrying a plurality of cams, the camshaft being mounted in a cam carrier and being arranged for a limited degree of axial movement and having associated with it means for effecting such movement, each of the cam surfaces having an outline, in a section plane containing the axis of the camshaft, which is not parallel to that axis, whereby in use the valve action is a function of the axial location of the camshaft within the range of permitted axial movement, the mechanism also comprising a cam follower for each cam, the cam follower comprising a one-piece body which reciprocates within a slideway and at one extremity acts upon the end of a valve stem through only a shim and has at the opposite extremity a trough of part-circular cross-section which receives a member in the form of a segment of a circular cylinder, the curved surface of which faces the interior surface of the trough, so that said member can turn with respect to said body, whilst a planar side surface of the member faces the cam surface. The present invention offers developments derived from this earlier valve control mechanism.
According to one aspect of the present invention, there is provided, in or for use in an internal combustion engine, a valve control mechanism which comprises: a camshaft carrying a plurality of cams, the camshaft being mounted in, or being adapted to be mounted in, a cylinder head or cam carrier, the valve control mechanism further including means for relatively advancing and retarding the rotation of the camshaft, said advance/retard means comprising a piston housed and axially displaceable within a cylinder, the axial position of said piston being under hydraulic control, and a mechanical coupling between said piston and the camshaft, said mechanical coupling serving to translate the axial movement of said piston into relative rotational movement of said camshaft.
The piston and cylinder may be, housed within a camshaft pulley at the front end of the camshaft. In one embodiment, the cylinder is defined, at its front end, by a front plate having an annular flange extending towards the camshaft; and at its rear end by the front face of a housing within which said mechanical coupling is housed.
Advantageously, the mechanical coupling between the piston and the camshaft comprises a spline mechanism acting between said piston and the front end of said camshaft. Such a spline mechanism may be mounted within a housing, the front end surface of said housing forming the rear wall of said cylinder, as indicated above.
It will be appreciated that the configuration described is such that axial displacement of said piston causes a corresponding degree of rotational advancement of the camshaft when the axial movement is in one direction and a limited degree of rotational retardation of the camshaft when the axial movement is in the opposite direction.
Preferably, a first channel is provided to deliver hydraulic fluid to the interior of said cylinder directly behind said front plate, and a second channel is provided to deliver hydraulic fluid to said cylinder directly in front of the housing for said mechanical coupling.
In certain currently preferred embodiments of the invention, the piston is arranged so that its own axial displacement results additionally in axial displacement of the camshaft.
In a valve control mechanism in accordance with the invention, each cam generally has associated therewith a cam follower. Preferably, each cam follower comprises a body which reciprocates within a slideway and at one extremity acts upon the end of a valve stem, the cam follower having at its opposite extremity a trough of curved cross-section which receives a member in the form of a segment having on one side thereof a surface curved correspondingly to that of said trough, and having on the other side thereof a planar surface, whereby the curved surface of the segment enables said member to turn with respect to said body, while the planar surface of the member cooperates with the cam surface.
Preferably, the cam followers are disposed relative to the valve stems such that the One of action between the each cam follower and the end of its respective valve stem is located away from the mid-point (measured in a direction parallel to the axis of the camshaft) of a section through the cam fallower in a plane which contains the axis of the camshaft and the axis of the valve stem. A particularly preferred arrangement is where said end of the valve stem is partly recessed within the body of the cam follower.
In this first aspect of the invention, and in other aspects also, the hydraulic control of the piston action is preferably under microprocessor control.
According to a second aspect of the present invention, there is provided, in or for use in an internal combustion engine, a valve control mechanism which comprises: a camshaft carrying a plurality of cams, the camshaft being mounted in a cylinder head or cam carrier and being arranged for a limited degree of axial movement and having associated with it means for effecting such movement, each of the cam surfaces having an outline in section taken through a plane containing the axis of the camshaft which is not parallel to that axis, whereby in use the valve action is a function of the axial location of the camshaft within the range of permitted axial movement; and a cam follower for each cam, the cam follower comprising a body which reciprocates within a slideway and at one extremity acts upon the end of a valve stem, the cam follower having at its opposite extremity a trough of part-circular cross-section which receives a member in the form of a segment of a circular cylinder, the curved surface of which faces the interior surface of the trough, so that said member can turn with respect to said body; while a planar side surface of the member faces the cam surface,
In this second aspect, the valve control mechanism may further comprise means for relatively advancing and retarding the rotation of the camshaft.
In one embodiment of this second aspect of the invention, control of the axial displacement of the camshaft acts also to control the relative rotational adjustment of the camshaft. This provides good control of valve action and can be implemented relatively easily. In an alternative embodiment, control of the axial displacement of the camshaft acts independently of the relative rotational adjustment of the camshaft. This permits greater freedom to influence valve action, but requires more control functions within or associated with the engine.
Advantageously, the means for effecting the limited degree of axial displacement of the camshaft comprises a piston housed within a cylinder, the axial position of the piston being under hydraulic control. Such hydraulic control of said piston is preferably governed by a microprocessor. This may be achieved, for example, by use of oil supplied by a proportional programmable valve such as xe2x80x9cMoogxe2x80x9d valve under microprocessor control.
In one implementation of this aspect of the invention, the piston can advantageously act on the camshaft to effect axial displacement thereof through a spline mounted within a housing, the arrangement being such that axial displacement of the piston causes one or both of: (a) a corresponding degree of axial displacement of the spline and of the camshaft; and (b) a limited degree of rotational advancement of the camshaft when the axial movement is in one direction and a limited degree of rotational retardation of the camshaft when the axial movement is in the opposite direction. The spline may for example be a straight spline or a ball spline.
In order to reduce the bulk of an engine incorporating a valve control mechanism of this invention, the piston, cylinder and spline may be positioned at the front end of the camshaft, the conventional front end camshaft bearing being modified as will be described herein.
It will be appreciated that the features described above permit control of the valve lift and duration with different cam profiles varying infinitely within two limits (through the axial displacement of the camshaft) and of the valve timing (through the rotational adjustment of the camshaft). When these three functions operate together, the duration of valve opening, their angular shift and the envelope of the curve obtained by plotting valve position against time
In a further embodiment, the profile of each cam is such that a line connecting the points of maximum radial extent of the cam at opposite ends (in the to the axis of the camshaft. Cams of this type of profile are described as swashed cams. When viewed in the direction of the camshaft axis, cams of this sort display a phase angle between the camshaft axis and the line marking the xe2x80x9cnosesxe2x80x9d of the cam profile. Using cams of this structure allows more extended control of valve action, in particular timing, when the camshaft undergoes axial displacement.
Another aspect of the present invention addresses the structure of cam follower used, in particular its cooperation with the end of the valve stem. Accordingly, the present invention also provides, in or for use in an internal combustion engine, a valve control mechanism which comprises: a camshaft carrying a plurality of cams, the camshaft being mounted in a cylinder head or cam carrier and being arranged for a limited degree of axial movement and having associated with it means for effecting such movement, each of the cam surfaces having an outline in section taken through a plane containing the axis of the camshaft which is not parallel to that axis, whereby in use the valve action is a function of the axial location of the camshaft within the range of permitted axial movement; and a cam follower for each cam the cam follower comprising a body which reciprocates within a slideway and at one extremity acts upon the end of a valve stem, the cam follower having at its opposite extremity a trough of part-circular cross-section which receives a member in the form of a segment of a circular cylinder, the curved surface of which faces the interior surface of the trough, so that said member can turn with respect to said body, while a planar side surface of the member faces the cam surface; wherein the cam followers are disposed relative to the valve stems such that the zone of action between each cam follower and the end of its respective valve stem is located away from the mid-point (measured in a direction parallel to the axis of the camshaft) of a section through the cam follower in a plane which contains the axis of the camshaft and the axis of the valve stem: The end of the valve stem may be partly recessed within the body of the cam follower.
It will be appreciated that, in use, lubrication will be fed to the member in the form of a segment of a circular cylinder to reduce frictional effects when the member moves with respect to the trough in which it sits.
The invention also provides an internal combustion engine in which there is a camshaft carrying a plurality of cams and a cam follower for each cam, the cam follower comprising a body which reciprocates within a slideway and at one extremity acts upon the end of a valve stem, wherein the cam followers are disposed relative to the valve stems such that the zone of action between each cam follower and the end of its respective valve stem is located away from the mid-point (measured in a direction parallel to the axis of the camshaft) of a section through the cam follower in a plane which contains the axis of the camshaft and the axis of the valve stem. Preferably, the zone of action is located close to one side of the cam follower.
There are several unexpected advantages in utilising a cam follower of the type defined above; firstly, this type of mechanism packaging allows this device to operate within a currently packaged conventional engine size envelope. This in turn:-means that valve control mechanisms of this invention can be incorporated into standard production engines with minimum modification.
Secondly, the tendency of conventional cam profiles and followers to become damaged due to the force of the valve stem acting on the follower surface in such a way as to tip it relative to the cam lobe and produce uneven elastic deformation, leading to much higher local stresses and hence wear, is avoided due to improved cam/follower contact.
Thirdly, the cam follower can be configured to maximise its structural rigidity/mass ratio, thereby permitting improved valve gear and engine performance.
A valve control mechanism in accordance with this aspect of the invention can have the end of the valve stem partly recessed within the body of the cam follower.
According to a fourth aspect of the present invention, there is provided, in or for use in an internal combustion engine, a valve control mechanism which comprises: a camshaft carrying a plurality of cams, the camshaft being mounted in a cylinder head or cam carrier and being arranged for a limited degree of axial movement and having associated with it means for effecting such movement, each of the cam surfaces having an outline in section taken through a plane containing the axis of the cammshaft which is not parallel to that axis, whereby in use the valve action is a function of the axial location of the camshaft within the range of permitted axial movement; and a cam follower for each cam, the cam follower comprising a body which reciprocates within a slideway and at one extremity acts upon the end of a valve stem, the cam follower having at its opposite extremity a trough of part-circular cross-section which receives a member in the form of a segment of a circular cylinder, the curved surface of which faces the interior surface of the trough, so that said member can turn with respect to said body, while a planar side surface of the member faces the cam surface; the valve control mechanism further comprising means for relatively advancing and retarding the rotation of the camshaft, and wherein the cam followers are disposed relative to the valve stems such that the zone of action between each cam follower and the end of its respective valve stem is located away from the mid-point (measured in a direction parallel to the axis of the camshaft) of a section through the cam follower in a plane which contains the axis of the camshaft and the axis of the valve stem.
According to a fifth aspect of the present invention, there is provided a method of controlling the valves in an internal combustion engine, which comprises supplying a hydraulic fluid under the control of a microprocessor to cause relative advancement and retardation of the rotation of the camshaft. Such a method permits continuously variable advance/retard control.
In this fifth aspect of the invention, the hydraulic fluid is preferably caused to act on a piston housed within a cylinder, the axial displacement of said piston acting upon the camshaft to cause relative advancement and/or retardation thereof. The piston may be arranged so that its own axial displacement results additionally in axial displacement of the camshaft. Conveniently, the piston is disposed within a camshaft pulley.
The disposition of the piston and cylinder within a camshaft pulley at the front end of an internal combustion engine provides a very effective mounting for the camshaft as well as facilitating the control functions which may be required (e.g. relative advance/retard and/or axial displacement of the camshaft).
According to a sixth aspect of the present invention, there is provided, in or for use in an internal combustion engine, a cam follower which comprises a body which, in use, reciprocates within a slideway within the engine and at one extremity acts upon the end of a valve stem, the cam follower comprising a generally rectangular, hollow body adapted at its upper end for contact with a cam and having at its lower end a contact surface for cooperation with the valve stem, characterised in that said contact surface is located so as to be contiguous with a side wall of the hollow body.
According to a seventh aspect of the present invention, there is provided, in or for use in an internal combustion engine, a cam follower which comprises a body which, in use, reciprocates within a slideway within the engine and at one extremity acts upon the end of a valve stem, the cam follower comprising a generally rectangular, hollow body adapted at its upper end for contact with a cam and having at its lower end a contact surface for cooperation with the valve stem, characterised in that the zone of action between the cam follower and the end of its respective valve stem is a contact surface which is located away from the mid-point (measured in a direction parallel to the axis of the camshaft) of a section through the cam follower in a plane which contains the axis of the camshaft and the axis of the valve stem.
Advantageously, a cam follower as defined above includes an internal wall extending upwardly from the lower end of the hollow body and parallel to the shorter sides of the body midway between the shorter sides, said internal wall including, at is junction with one of the longer sides of the body, the contact surface which, in use, makes contact with the valve stem.